全文获取类型
收费全文 | 259846篇 |
免费 | 4348篇 |
国内免费 | 3403篇 |
专业分类
测绘学 | 7042篇 |
大气科学 | 19165篇 |
地球物理 | 54356篇 |
地质学 | 90143篇 |
海洋学 | 21415篇 |
天文学 | 56421篇 |
综合类 | 1016篇 |
自然地理 | 18039篇 |
出版年
2021年 | 2255篇 |
2020年 | 2612篇 |
2019年 | 2869篇 |
2018年 | 3595篇 |
2017年 | 3214篇 |
2016年 | 5784篇 |
2015年 | 4237篇 |
2014年 | 6954篇 |
2013年 | 14289篇 |
2012年 | 6541篇 |
2011年 | 7909篇 |
2010年 | 6863篇 |
2009年 | 9583篇 |
2008年 | 8400篇 |
2007年 | 7814篇 |
2006年 | 9681篇 |
2005年 | 7767篇 |
2004年 | 7676篇 |
2003年 | 7155篇 |
2002年 | 6800篇 |
2001年 | 6041篇 |
2000年 | 5964篇 |
1999年 | 5214篇 |
1998年 | 5243篇 |
1997年 | 5051篇 |
1996年 | 4708篇 |
1995年 | 4445篇 |
1994年 | 4129篇 |
1993年 | 3863篇 |
1992年 | 3653篇 |
1991年 | 3614篇 |
1990年 | 3774篇 |
1989年 | 3528篇 |
1988年 | 3320篇 |
1987年 | 3849篇 |
1986年 | 3417篇 |
1985年 | 4240篇 |
1984年 | 4752篇 |
1983年 | 4431篇 |
1982年 | 4336篇 |
1981年 | 3949篇 |
1980年 | 3661篇 |
1979年 | 3521篇 |
1978年 | 3503篇 |
1977年 | 3293篇 |
1976年 | 3054篇 |
1975年 | 2963篇 |
1974年 | 2932篇 |
1973年 | 3081篇 |
1972年 | 2026篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
The existence of prolonged periods of abnormally low solar activity (such as the Maunder minimum) is explained within the framework of Leighton's model of a solar cycle with a hypothetical internal magnetic field of the Sun taken into account. 相似文献
212.
The paper considers the random phase errors in the phase shifters which are used in an antenna array to steer the beam in the look direction, and analyzes the effect of these errors on the performance of the optimal processor which maximizes the output SNR by deriving the expressions for the output signal power, residual interference power, output SNR, and the array gain as a function of the variance of these errors. The paper also considers the phase quantization error which arises in the digital phase shifters and shows how the performance of the optimal processor depends on the number of bits of the digital phase shifters. 相似文献
213.
214.
Michael K. WEISBERG Harold C. CONNOLLY Denton S. EBEL 《Meteoritics & planetary science》2004,39(10):1741-1753
Abstract— Amoeboid olivine aggregates (AOAs) are irregularly shaped, fine‐grained aggregates of olivine and Ca, Al‐rich minerals and are important primitive components of CR chondrites. The AOAs in CR chondrites contain FeNi metal, and some AOAs contain Mn‐rich forsterite with up to 0.7 MnO and Mn:Fe ratios greater than one. Additionally, AOAs in the CR chondrites do not contain secondary phases (nepheline and fayalitic olivine) that are found in AOAs in some CV chondrites. The AOAs in CR chondrites record a complex petrogenetic history that included nebular gas‐solid condensation, reaction of minerals with the nebular gas, small degrees of melting, and sintering of the assemblage. A condensation origin for the Mn‐rich forsterite is proposed. The Mn‐rich forsterite found in IDPs, unequilibrated ordinary chondrite matrix, and AOAs in CR chondrites may have had a similar origin. A type A calcium, aluminum‐rich inclusion (CAI) with an AOA attached to its Wark‐Lovering rim is also described. This discovery reveals a temporal relationship between AOAs and type A inclusions. Additionally, a thin layer of forsterite is present as part of the Wark‐Lovering rim, revealing the crystallization of olivine at the end stages of Wark‐Lovering rim formation. The Ca, Al‐rich nodules in the AOAs may be petrogenetically related to the Ca, Al‐rich minerals in Wark‐Lovering rims on type A CAIs. AOAs are chondrite components that condensed during the final stage of Wark‐Lovering rim formation but, in general, were temporally, spatially, or kinetically isolated from reacting with the nebula vapor during condensation of the lower temperature minerals that were commonly present as chondrule precursors. 相似文献
215.
Recently, exact spatially-homogeneous solutions have been found for a conformally invariant massless scalar fieldS(t) coupled to a Robertson-Walker geometry. We investigate extending these solutions to inhomogeneous scalar fieldsS(t, x
l
). For an isotropic scalar fieldS(t, r) we find a solution satisfying three of the five field equations. If we use REDUCE, we show that the remaining equations forceS=S(t). 相似文献
216.
G. Djurašević P. Rovithis H. Rovithis-Livaniou E. Fragoulopoulou 《Astrophysics and Space Science》2005,296(1-4):311-314
BV light curves of the eclipsing binary UV Leo obtained at the Kryonerion Astronomical Station of the National Observatory of
Athens, Greece, are analyzed. The analysis is based on a Roche configuration with two spots on the secondary surface. The
elements of the two components of the system are calculated and the spot characteristics are given. 相似文献
217.
218.
J. CAMPSIE M. H. RASMUSSEN L. C. KOVACS F. DITTMER J. C. BAILEY N. O. HANSEN J. LAURSEN L. JOHNSON 《Polar research》1990,8(2):237-243
New aeromagnetic data, K-Ar age determinations of dredged marine igneous rocks, as well as other geophysical evidence have shed light on the chronology, nature and evolution of the northern Iceland Plateau. Correspondence between seismic refraction profiles taken on the Jan Mayen Ridge and westward through Jan Mayen Island, suppressed aeromagnetic anomalies, earthquake surface wave studies, and ages of dredged igneous rocks suggest these strata may form an extended region of thickened crust, possibly of Caledonian age, extending westward toward the Kolbeinsey Ridge and northwest to the south wall of the Jan Mayen Fracture Zone. 相似文献
219.
220.